Protein sub-unit vaccines are the safest general strategy to induce long-term immune memory. In these vaccination regimens, the quality and quantity of immunity is determined by non-specific immune adjuvants. Unfortunately, how these immune adjuvants work in the vaccine recipient remains poorly understood. We have developed novel means for studying immune adjuvants in small animal models to determine how they impact innate immunity, Th cells and B cells specific for protein antigens. These approaches will provide information on the mechanism of adjuvanticity in vivo and help us to design improved adjuvants for future protein-based vaccines. Innate regulation of adaptive immunity is the primary focus of this current application. Specific recognition of foreign peptide-MHCII expressing DC by naive Th cells constitutes the earliest 'cognate'checkpoint in the development of Th cell regulated B cell immunity. We recently provided evidence that Th cell clonal selection is based on threshold levels of TCR-pMHCII affinity. In preliminary studies, we find that changing the immune adjuvant at initial priming, substantially alters TCR repertoire selection. These data led us to hypothesize that adjuvant controls DC maturation and function in ways that alter TCR selection thresholds, impact the programming of effector Th cell function, and alter the quality of B cell immunity. We will test aspects of this hypothesis across two specific aims. In the first aim, we will use an animal model with high resolution for the analysis of TCR repertoires to determine how adjuvant controls Th clonal selection and development of effector function in vivo. In the second aim, we develop a new animal model to evaluate the impact of adjuvant on the development and function of pMHCII+ DC and how this impacts the quality of the adaptive immunity. These studies will help to define and refine adjuvant action in vivo and may identify new molecular targets for novel adjuvants and immunotherapeutics in the preventative and clinical management of infectious disease.